The statement that many of us walk around every day not fully hydrated is a correct one. The average person should drink 80oz of water everyday and you should drink anextra 10oz glass for ever 25lbs that your overweight. That much water is just not something the average Teen is going to drink let alone the average adult. I can see how a person can fail a hydration test and not even know it. The average student does not have the ability(Time) to drink that much water in a days time at school.
At the International Sports Medicine Institute, where they work with Olympic and professional athletes from around the world, they have developed a formula for water intake that accommodates athletes and non-athletes alike. They suggest a daily water intake of 1/2 ounce per pound of body weight if you're a non-active person (that's 10 eight-ounce glasses a day if your weight is 160 pounds), and 2/3 ounce per pound if you're an active 160 pounds). This ISI formula, inspired by East Germany physicians, has been used with great success for almost two decades.
So if your are a 210 pounder and an athlete you should drink a gallon of water a day plus an extra 10oz glass
link to hydration calculator.
http://www.bottledwater.org/public/hydratio_main.htm